Multi-inlet mass spectrometer

ABSTRACT

A mass spectrometer has an ion source ( 10 ) with a plurality of atmospheric pressure sample ioniser ( 20 ) mounted in a front face ( 15 ) thereof. Each sample ioniser ( 20 ) extends into a corresponding sample region ( 30 ) and the tip of each sample ioniser is mounted at right-angles to a corresponding one of a plurality of entrance cones ( 50 ) each having an entrance orifice ( 40 ) therein. Each entrance cone ( 50 ) in turn opens into an inlet channel having first and second parts ( 60, 70 ). The two parts of the inlet channel are separated by an electrical gate ( 65 ). The inlet channels corresponding to each entrance cone ( 50 ) all merge into a common exit channel ( 90 ) to a mass spectrometer. By appropriate operation of the gates ( 65 ) dividing the inlet channels, rapid switching between the samples that are analysed in the mass analyser can be achieved.

This application claims benefit, under 35 U.S.C. 371, of PatentCooperation Treaty PCT/GB01/03368, filed Jul. 26, 2001, which claimsbenefit of Great Britain priority application Number 0018344.2, filedJul. 26, 2000.

FIELD OF THE INVENTION

The present invention relates to an ion source for a mass spectrometer,in particular of the type adapted to provide a source of sample ions atatmospheric pressure.

BACKGROUND TO THE INVENTION

Mass spectrometers have been used to analyse a wide range of materials,including organic substances such as pharmaceutical compounds,environmental compounds and biomolecules. They are particularly useful,for example, for DNA and protein sequencing. In such applications, thereis an ever increasing desire for faster speed of analysis of sample ionsby the mass spectrometer while still producing accurate measurements ofthe mass/charge ratios of the ions in question.

Some steps towards increasing the speed of analysis of massspectrometers have been taken by increasing the number of inlets to themass spectrometer. For example, in Analytical Chemistry 2000, 72, pages20-24, L. Jiang and M. Moini describes a dual- or quad-orifice massspectrometer which receives sample ions from two or four electrosprayionisation sources respectively. In this way, several streams of sampleions can be analysed simultaneously and furthermore a stream ofreference ions can be introduced into the mass spectrometer at the sametime as the stream of sample ions, enabling more accurate readings.

An alternative arrangement is shown in EP-A-0,966,022. A massspectrometer is shown having a single sampling orifice for receivingions and a plurality of sample ion sources. The sampling orifice isconnected to a sample selector with at least one aperture. Each ionsource generates a jet of ions directed towards the sample selectorwhich may rotate to align an aperture with any one of the jets. In thismanner any one of a number of different jets of sample ions to beanalysed may enter the sample selector, pass through the samplingorifice and into the mass spectrometer.

U.S. Pat. No. 6,066,848 provides, in a first embodiment, an ion sourcehaving a plurality of sample ionisers and in which a rotatable dischaving a small hole is provided adjacent the inlet orifice of a massspectrometer. Rotation of the disc allows a selected one of the streamsof sample ions to enter the analyser. In a second embodiment, the discis stationary and is provided with number of shutter valves which may beindividually opened or closed to permit one of the streams of sampleions to enter the analyser.

The prior art mass spectrometers described above all suffer from variousdrawbacks. For example, in the case of the arrangement described byJiang and Moini, providing a plurality of orifices leading into the massspectrometer requires extra pumps to maintain a suitably low pressurewithin the mass spectrometer, especially with the quad-nozzlearrangement. In the case of the devices shown in EP-A-0,966,022 and thefirst embodiment of U.S. Pat. No. 6,066,848, the rotating sampleselectors are both cumbersome, slow to react, and also difficult toengineer reliably. Finally, none of these devices are particularly fastin switching from one sample stream to the next.

It is an object of the present invention to address these problems withthe prior art.

SUMMARY OF THE INVENTION

Accordingly, in a first aspect of the present invention there isprovided an ion source for a mass spectrometer which operates at lowpressure, the ion source comprising: a plurality of atmospheric pressuresample ionisers operative at relatively higher pressure to provide aplurality of streams of sample ions; an interface chamber, operable at apressure intermediate atmospheric pressure and the operating pressure ofthe mass spectrometer, having a plurality of entrance orifices locatedat a first position to collect sample ions into the interface chamberfrom said streams of sample ions and an exit orifice at a secondposition downstream of the said first position of the entrance orifices,for said sample ions to exit the interface chamber to the massspectrometer, the interface chamber defining a plurality of separate ionpaths for ions to travel between a respective one of the entranceorifices and the exit orifice; and ion control means, located downstreamof the said entrance orifices, and arranged selectively to prevent ionsfrom passing along a chosen one or ones of the ion paths to the massspectrometer.

By providing an ion source having a plurality of entrance orifices tothe interface chamber, it is possible to separately analyse any one of anumber of different streams of sample ions, such as protein molecules orDNA fragments. Furthermore, as the sample ionisers may each beconstantly producing a stream of sample ions, it is possible to rapidlyswitch to and analyse the next stream of sample ions simply by ‘closing’the, or part of the, ion control means and ‘opening’ another partthereof. This increases the rate at which a large number of samples maybe analysed, and greatly increases the speed of analysis with a singlemass spectrometer.

Either one or any multiple combinations of the plurality of streams ofsample ions may be admitted to the mass spectrometer for simultaneousstudy. A further advantage of the present invention becomes apparentwhen more than one stream of sample ions is admitted to the massspectrometer: as the sample ions only mix within the interface chamberwhich is at a pressure lower than atmospheric pressure, the chance ofcollision and, as a result, the rate of chemical reaction between thedifferent sample ions is greatly reduced. This ensures that the massspectrometer receives as few unwanted or unexpected chemical compoundsas is possible and produces accurate results.

In order for a stream of sample ions to enter the interface chamber, itmust each pass through one of the entrance orifices. As a result, thereare fewer sample ions and, more importantly, fewer unwanted chemicalcompounds within the interface chamber than in the region immediatelysurrounding the sample ioniser. By providing the ion control means, orion blocking means, downstream of the entrance orifices, therefore, theyare less likely to become clogged or otherwise damaged by the unwantedchemicals entrained within the sample stream.

In another preferred embodiment, during use of the ion source, theinterface chamber is maintained at a pressure intermediate atmosphericpressure and the operating pressure of the mass spectrometer. Thisfurther increases the speed of analysis by the mass spectrometer as theion control means are arranged downstream of the entrance orifices ofthe interface chamber. The ion streams thus encounter the ion controlmeans in a region of relatively low pressure. In this region, the sampleions travel at substantially greater speeds than in the relativelyhigher pressure region immediately surrounding the sample ioniser. As aresult, when the, or part of the ion control means is ‘closed’, andanother part ‘opened’, the time required for the next stream of sampleions to reach the mass spectrometer is reduced. The relaxation timebetween one stream and the next may be thus reduced by a factor of tencompared to the prior art (10 ms as compared to the 100 ms for thesystem described in EP-A-0,966,022).

In one preferred embodiment, the ion control means includes gating meanswhich, when open, permits passage of a selected one or ones of streamsof sample ions to the mass spectrometer, the gating means being providedwithin the interface chamber between the said first and secondlocations. In that case, preferably, the gating means comprises anelectromagnetic field generator arranged selectively to generate anelectromagnetic field which deflects the selected one or ones of thestreams of sample ions so as to prevent the or each said stream ofsample ions from entering the mass spectrometer. In a particularlypreferred embodiment, the electromagnetic field generator generates astatic electric field. Non-mechanical switching provides a further speedadvantage over the prior art. Not only that, but electrical gates aremore reliable and easier to install into present systems. For example, apair of electrodes generating an electric field may be placed around theinterface chamber. Deflection of sample ions may thus be achievedwithout major modification to the interface chamber. Furthermore,electrical gates are cleaner and also easier to keep clean than theirmechanical equivalents. For example, the stream of sample ions may wellcontain other, unwanted chemicals such as solutes and buffers, and thesecan collect onto and clog mechanical gates. This forces regular cleaningof the gates, or otherwise reduces their lifespan. Also, these unwantedchemical deposits may later break free from the gate, contaminatingother sample flows.

In an alternative embodiment, the ion control means comprises iontrapping means arranged selectively to prevent ions entering it fromexiting therefrom. In that case, the interface chamber defines aplurality of interface channels each in communication with acorresponding one of the entrance orifices, each interface channel inturn constraining a corresponding one of said streams of sample ions tofollow a corresponding one of the said ion paths, preferably.

Most preferably, the ion trapping means comprises a plurality of ionstorage devices, such as for example rf multipole storage devices, eachbeing arranged to recieve a stream of sample ions from a correspondingone of the said seperate ion paths and selectively to trap the receivedstream therein for future ejection to the exit orifice on demand.

Using ion traps to store the ions arriving from multiple sourcesprovides yet a further improvement in device duty cycle, particularlywhen non-electrospray sources are employed.

In yet a further preferred embodiment at least one of the plurality ofsample ionisers provides a stream of ions for calibrating the massspectrometer, the stream of ions for calibrating the mass spectrometerbeing admitted to the mass spectrometer simultaneously with at least oneother of the streams of sample ions. By admitting a stream of sampleions to the mass spectrometer, either sequentially or simultaneouslywith a stream of calibration ions, on-line calibration can be providedand the accuracy of the mass spectrometer increased. Furthermore, as thetwo streams of ions mix only within the relatively lower pressure regionof the interface chamber, fewer chemical reaction will occur between thecomponent ions than in the ion source of the prior art.

It is to be understood that whilst a separate, distinct sample may befed to a respective one of the plurality of ionisers, any combination ofsamples may in fact be used. In particular, it may be beneficial forsensitivity improvement to split the same sample into two or moreionisers, for feeding to two or more separate channels.

In another preferred embodiment of the present invention, the interfacechamber is arranged in fixed relation to the sample ionisers. Previousion sources (such as EP-A-0,966,022) have included an interface chamberwhich rotates relative to the sample ionisers in order to select therequired stream of sample ions. The present invention, by providing afixed interface chamber, provides a system which is more reliable andeasier to engineer.

In another aspect, the present invention provides a method of analysingsample ions from at least one of a plurality of simultaneously operatingatmospheric pressure sample ionisers, the method comprising: generatinga stream of sample ions from each of a corresponding one of theplurality of atmospheric pressure sample ionisers; directing each streamtowards a corresponding one of a plurality of entrance orifices in aninterface chamber, maintained at a pressure below atmospheric pressure,for selective direction through the interface chamber towards a massspectrometer; and selectively blocking at least some of said streams ofsample ions from passing through said exit orifice of said interfacechamber into the mass spectrometer after said selected one or ones ofsaid streams of sample ions have entered said interface chamber.

Further advantageous features are set out in the dependent claimsattached hereto.

BRIEF DESCRIPTION OF THE FIGURES

One embodiment of the present invention will now be described by way ofan example only and with reference to the accompanying drawings inwhich:

FIG. 1 shows a side cross-sectional view of an ion source embodying thepresent invention;

FIG. 2 shows a section along the line AA′ of FIG. 1; and

FIG. 3 shows a side cross-sectional view of an alternative ion sourceembodying the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring first to FIG. 1, an ion source, generally indicated at 10, isshown.

The ion source 10 has a front face 15 and includes a plurality ofatmospheric pressure sample ionisers 20, mounted therein. A variety ofdifferent ionisers are suitable, such as an electrospray ion source, anatmospheric pressure chemical ionisation (APCI) ion source or amatrix-assisted laser desorption/ionisation (MALDI) ion source. As willbe familiar to those skilled in the art, the ioniser 20 is provided witha flow of solvent containing a sample to be analysed. Typically, thisflow is produced by separating the sample molecules by liquidchromatography or capillary electrophoresis. However, other techniquessuch as fast liquid chromatography and capillary electrochromatographycan be used as well.

Each ioniser 20 extends into a corresponding sample region 30, which isagain at or around atmospheric pressure. The sampling region 30 isdefined between the end of each ioniser 20 and an entrance orifice 40 inan entrance cone 50. As will be understood, the tip of each ioniser isarranged at right-angles to the entrance orifice of the correspondingentrance cone 50, so that sample ions and entrained solvent moleculesare not forced directly into the entrance orifice 40.

Each entrance cone 50 communicates with a corresponding inlet channelwhich has a first part 60 and a second part 70 defined in an interfacechamber 80. The first part 60 of the inlet channel meets the second part70 of the inlet channel at an oblique angle as may be seen in FIG. 1. Atthe junction between the two parts of each inlet channel is anelectrical gate 65, whose purpose will be described in detail below.

Each inlet channel opens into a common exit channel 90, also defined inthe interface chamber 80. Adjacent to the common exit channel 90 is anexit orifice 100 in an exit cone 110. The exit orifice allows ionswithin the common exit channel 90 to pass therethrough and into a massspectrometer (not shown).

The common exit channel 90 opens into a pumping chamber 120 to which isconnected a vacuum pump, typically a rotary pump (not shown). In thismanner, the pressure in the interface chamber 80, between the entranceorifices 40 and the exit orifice 100, is maintained below atmosphericpressure, typically around 10 to 15 mBar.

In prior art ion sources having only a single entrance orifice such asare described in WO 98/49710, although the pressure at the exit orificeis about 10 to 15 mBar, the slow rate of gas flow means that thepressure on the pump is only about 1 mBar. In the system of WO 98/49710,a small restrictor is used to reduce pump efficiency and to maintain therequired pressure. The same pump can then also be used as a backup pumpto the more powerful turbo pumps which maintain the mass spectrometer atan even lower pressure (typically about 10⁻⁴ mBar).

In a preferred embodiment of the present invention, the restrictor isremoved to offset the increased gas flow rate caused by the introductionof a plurality of entrance orifices 40. In this way, the requiredpressure in the interface chamber 80 may be maintained without theintroduction of a second vacuum pump. However, it may be necessary for asystem having 8 to 10 entrance orifices 40, for example, to provide afurther, lower pumping speed pump to act as a backup to the turbo pumps.

Turning now to FIG. 2, a section along the line AA′ of FIG. 1 is shown.FIG. 2 illustrates the layout of the plurality of inlet channels andentrance cones which are fed by the corresponding plurality of ionisers.As seen in that Figure, eight inlet channels are arranged in a circle,to allow samples from eight different ionisers to be received. Eachentrance cone 50 receives samples from a corresponding sample ioniser,and these pass into a corresponding first part 60 of a correspondinginlet channel. For example, the entrance cone labelled 50A opens into afirst part (not shown in FIG. 2) of the inlet channel. This in turnleads into a second part 60A of the inlet channel. Adjacent inletchannels are separated by ribs 130.

As may be appreciated through considering FIGS. 1 and 2 in combination,the second parts 70 of the separate inlet channels converge at arelatively shallow angle, meeting at the common exit channel 90. Thus,the eight second parts 70 of the inlet channel together form afrustoconical shape. The shallow angle between the inlet channels andthe common exit channel 90 prevents excessive turbulence in ions as theyapproach the exit orifice 100.

In use, each of the eight ionisers 20 typically supplies differentsample ions. However, it is to be appreciated that at least some of theionisers may in fact receive the same sample from the liquidchromatograph (for example). This could improve the sensitivity of thedevice.

In contrast to prior art devices, each of the ionisers 20 generatessample ions continuously, rather than being switched on and off asrequired. Thus, ions from each of the separate ionisers travel throughthe corresponding entrance orifices 40 in the entrance cone 50corresponding to that particular ioniser. The different sample ions thentravel down their own, separate inlet channels. In other words, absentan electrical gate 65 in each inlet channel, all eight different sampleswould arrive continuously, together, at the exit orifice 100.

The electrical gate 65 in each inlet channel is, as previously describedin connection with FIG. 1, located at the junction between the firstpart 60 and the second part 70 thereof. In the exemplary embodiment ofFIG. 1, the electrical gate 65 is formed from an electrode which iscapable of generating an electric field of suitable magnitude to deflectthe sample ions passing down the first part 60 of the inlet channel,onto the wall of the interface chamber 80. This prevents them frompassing along the second part 70 of the inlet channel and into thecommon exit channel 90.

Each of the eight electrodes mounted, separately, in the eight inletchannels, is connected to a common controller. This allows a user todetermine which of the samples is to be allowed to pass along the lengthof the inlet channel and into the common exit channel 90. In one mode,the electrodes are manually switchable such that, at a given time, theelectrical gates 65 in seven of the eight inlet channels are “closed”,and only one of the electrical gates 65 is “open”. In a second mode, thecontroller may automatically switch the electrical gates 65 in rapidsuccession such that successively different samples are admitted intothe common exit channel 90. In yet a further mode, two or even more ofthe electrical gates 65 may be open simultaneously. This would beuseful, for example, when species from separate flows are known not tointerfere in the mass spectrum and therefore the duty cycle could beincreased.

The bend in the inlet channel at the junction between the first andsecond parts thereof serves two purposes. Firstly, it avoids thepresence of a direct line of sight between any of the entrance orifices40 and the single exit orifice 100. This prevents “streaming” of sampleions from the entrance to the exit orifices, which is advantageous.Secondly, by locating the electrode to generate the electrical gate 65at that junction, the electric field shape is particularly efficient inpreventing ions from travelling through the inlet channel when theelectrical gate 65 is closed.

Although all eight ionisers 20 may provide ions from a sample to beexamined, it is preferable that one of the ionisers 20 is insteadprovided with a flow of solvent containing molecules which, whenionised, have a known mass/charge ratio. This is particularly useful toallow a mass spectrometer in communication with the exit orifice 100 tobe calibrated. In this case, the inlet channel fed by the calibrationioniser is typically left open (that is, the electrical gate 65 in thatchannel is opened) whilst a sample to be analysed (from another of theionisers) is admitted at the same time.

Gating of the different inlet channels allows for any combination of thedifferent streams of sample ions to be mass analysed. Further, the highspeed electrical gating enables fast switching from one stream of sampleions to the next, increasing the speed of analysis by the massspectrometer. Typically, the interface chamber 80 is maintained at apressure of around 10 to 15 mBar. Accordingly, the sample ions in theinlet channel are typically travelling at speeds of over 100 m/s incomparison to speeds of around 10 m/s in the relatively higher pressuresample region 30 surrounding the entrance orifice 40. With thisincreased speed of travel, when one of the electrical gates 65 is open,the relaxation time before the sample ions reach the exit orifice 100 isconsiderably shorter than in the prior art. This increases the switchingspeed, and hence the speed of analysis by the mass spectrometer, yetfurther.

Although a preferred embodiment of the invention has been described, itis to be understood that various modifications or alternatives arecontemplated. In particular, any number of sample ionisers 20, togetherwith a corresponding number of entrance cones 50 and inlet channels, maybe included, and these may be arranged in any suitable configuration.However, increasing the number of entrance orifices 40 will increase thepressure in the interface chamber 80 for a given pumping speed.

Furthermore, although in the preferred embodiment an electrical gate 65is employed in each inlet channel, at the junction between the first andsecond parts thereof, it would be appreciated that different techniquesmay be used for gating or blocking the ions. For example, a mechanicalgate such as a shutter valve could be used in place of an electricalgate generated by an electrode, to block the flow of sample ions througheach inlet channel. Furthermore, rather than using a static electricfield, it may be advantageous under certain circumstances to employ anRF field instead.

In an alternative embodiment of the present invention, shown in FIGS. 3and 4, instead of a plurality of electrical or electrically-operatedmechanical gates, the multiple ion paths from the ionisers via theinterface chamber to the mass spectrometer may instead be selectivelyblocked by a plurality of rf-only multipole storage devices (such as aquadrupole or hexapole arrangement). These are of themselves well knownand are shown, for example, in U.S. Pat. Nos. 5,420,425, 6,020,586 and5,179,278.

Referring to FIG. 3, two of a plurality of sample ionisers 20 (e.g.nanosprays) are shown, each extending into a corresponding samplingregion 30, and pointing directly at an associated entrance orifice 40 ofan entrance cone 50. It will be understood that, as with previousembodiments, each sample ioniser 20 may be arranged at right-angles toits associated entrance orifice 40. Each entrance cone 50 communicateswith a corresponding inlet channel 60, defining an ion path.

Electrodes of an rf-only multipole ion trap 65 are shown arranged aroundeach inlet channel 60. Preferably, one rf-only ion trap 65 is positionedin a corresponding one of the ion paths between the entrance orifices 40and the exit orifice 100, that is, a separate storage device 65 isprovided for each ion stream within the multipole arrangement.

The common exit channel 90 opens into a pumping chamber 120 to which isconnected a vacuum pump, typically a rotary pump (not shown).

During operation, the ions from a given ioniser 20, passing along thecorresponding inlet channel 60, are focussed onto the axis of the iontrap 65 associated with that inlet channel 60, even at relatively highpressures (several mBar). Ions may be trapped in each ion trap 65 byapplying a voltage to the end apertures or end-sections thereof and thestorage time may be up to a few seconds. Once trapped, ions may beejected by altering the ion trap parameters when desired. Thus, in adirectly analogous manner to the use of electrical or mechanical gates,ion traps 65 can simultaneously or sequentially supply a single streamof ions to a mass spectrometer from a multiple sample stream input. Theadvantage of this arrangement over the electrical/mechanical gatingtechnique is that the ion traps should provide a 100% duty cycle. Thisin turn permits higher sensitivity to be achieved.

1. An ion source for a mass spectrometer which operates at low pressure,the ion source comprising: a plurality of sample ionisers operative atatmospheric pressure to provide a plurality of streams of sample ions;an interface chamber, operable at a pressure lower than atmosphericpressure, having a plurality of entrance orifices located at a firstposition to collect sample ions into the interface chamber from saidstreams of sample ions and an exit orifice at a second positiondownstream of the said first position of the entrance orifices, for saidsample ions to exit the interface chamber to the mass spectrometer, theinterface chamber defining a plurality of separate ion paths for ions totravel between a respective one of the entrance orifices and the exitorifice, wherein each entrance orifice and the associated ion path arephysically separated from the other entrance orifices and ion pathsalong at least a portion of each of the ion paths; and ion controlmeans, located downstream of the said entrance orifices, and arrangedselectively to prevent ions from passing along a chosen one or ones ofthe ion paths to the mass spectrometer.
 2. An ion source as claimed inclaim 1, wherein the interface chamber further comprises an evacuationport and a vacuum pump connected to the evacuation port to maintain theinterface chamber at said pressure lower than atmospheric pressure. 3.An ion source as claimed in claim 1, in which the ion control meansincludes gating means which, when open, permits passage of a selectedone or ones of streams of sample ions to the mass spectrometer, thegating means being provided within the interface chamber between thesaid first and second positions.
 4. An ion source as claimed in claim 3,in which the gating means comprises an electromagnetic field generatorarranged selectively to generate an electromagnetic field which deflectsthe selected one or ones of said streams of sample ions so as to preventthe or each said stream of sample ions from entering the massspectrometer.
 5. An ion source as claimed in claim 4 wherein theelectromagnetic field generator generates a static electric field.
 6. Anion source as claimed in claim 3, in which the interface chamber definesa plurality of interface channels each in communication with acorresponding one of the entrance orifices, each interface channel inturn constraining a corresponding one of the said streams of sample ionsto follow a corresponding one of the said ion paths.
 7. An ion source asclaimed in claim 6, in which the interface chamber further defines asingle exit channel in communication with the exit orifice, theplurality of interface channels each converging into the single exitchannel.
 8. An ion source as claimed in claim 6, in which the gatingmeans comprises a plurality of individual gates, each gate beingassociated with a corresponding individual interface channel and beingarranged selectively to block a corresponding stream of sample ions,deriving from a corresponding one of the plurality of entrance orifices,from entering the mass spectrometer.
 9. An ion source as claimed inclaim 8, in which each gate extends across its corresponding interfacechannel in use, so as selectively to block ions passing along thatinterface channel.
 10. An ion source as claimed in claim 1, in which theion control means comprises ion trapping means arranged selectively toprevent ions entering it from exiting therefrom.
 11. An ion source asclaimed in claim 10, in which the interface chamber defines a pluralityof interface channels each in communication with a corresponding one ofthe entrance orifices, each interface channel in turn constraining acorresponding one of the said streams of sample ions to follow acorresponding one of the said ion paths.
 12. An ion source as claimed inclaim 11, in which the ion trapping means comprises a plurality of ionstorage devices each being arranged to receive a stream of sample ionsfrom a corresponding one of the said separate ion paths and selectivelyto trap the received stream therein for future ejection to the exitorifice on demand.
 13. An ion source as claimed in claim 12, in whicheach ion storage device comprises an if multipole ion storage device.14. An ion source as claimed in claim 6, in which at least some of theinterface channels are adapted so as to prevent a direct line of sightbetween their corresponding entrance orifice and the said exit orifice.15. An ion source as claimed in claim 14, in which the interfacechannels include a bend therein.
 16. An ion source as claimed in claim15, in which each gate is located adjacent to the bend in thecorresponding interface channel.
 17. An ion source as claimed in claim 1wherein at least one of said plurality of sample ionisers provides astream of ions for calibrating the mass spectrometer, said stream ofions for calibrating the mass spectrometer being admitted to the massspectrometer simultaneously with at least one other of said streams ofsample ions.
 18. An ion source as claimed in claim 1, wherein theinterface chamber is arranged in fixed relation to the sample ionisers.19. An ion source as claimed in claim 1, wherein the mass spectrometeris in communication with said exit orifice of the ion source.
 20. An ionsource as claimed in claim 3, in which the gating means comprisesplurality of electrically operated mechanical gates.
 21. A method ofanalysing sample ions from at least one of a plurality of simultaneouslyoperating atmospheric pressure sample ionisers, the method comprising:generating a stream of sample ions from each of a corresponding one ofthe plurality of atmospheric pressure sample ionisers; directing eachstream towards a corresponding one of a plurality of entrance orificesin an interface chamber, maintained at a pressure below atmosphericpressure, for selective direction through the interface chamber alongrespective separate ion paths defined by the interface chamber to travelbetween a respective one of the entrance orifices and an exit orificetowards a mass spectrometer; and selectively blocking at least some ofsaid streams of sample ions from passing through said exit orifice ofsaid interface chamber into the mass spectrometer after said selectedone or ones of said streams of sample ions have entered said interfacechamber, wherein selectively blocking the at least some of said streamsof sample ions does not affect streams of sample ions that are not to beblocked.
 22. A method as claimed in claim 21, wherein the massspectrometer is operative at a low pressure, the method furthercomprising maintaining the pressure within the interface chamber at apressure intermediate atmospheric pressure and the operating pressure ofthe mass spectrometer.
 23. A method as claimed in claim 21, wherein saidstep of selectively blocking at least some of said streams of sampleions comprises generating an electromagnetic field for deflectingselected one or ones of said streams of sample ions so as to prevent theor each said stream from reaching the mass spectrometer.
 24. A method asclaimed in claim 23 wherein said step of generating an electromagneticfield comprises generating a static electric field.
 25. A method asclaimed in claim 21,in which the step of selectively blocking at leastsome of the streams of sample ions comprises trapping selected ones ofthe said streams in a corresponding one of a plurality of ion traps. 26.A method as claimed in claim 25, further comprising subsequentlyejecting ions stored in a selected one or ones of the said ion traps anddirected the ejected ions towards the mass spectrometer.
 27. A method asclaimed in claim 21 further comprising providing a stream of ions forcalibrating the mass spectrometer and admitting said stream of ions forcalibrating the mass spectrometer to the mass spectrometersimultaneously with at least one other of said streams of sample ions.28. A method as claimed in claim 21, further comprising supplying atleast two of the plurality of atmospheric pressure sample ionisers withthe same sample to be ionized.
 29. A method as claimed in claim 21,wherein the interface chamber is arranged in fixed relation to thesample ionisers.